Human ZAQ ligand-1 (a polypeptide having the amino acid sequence represented by SEQ ID NO: 1 or SEQ ID NO: 2) (hereinafter sometimes briefly referred to as human ZAQL-1) is a ligand for the ZAQ receptor and is a peptide having the ileum contractile action (WO 02/06483). Also, human Bv8 mature peptide (hereinafter sometimes briefly referred to as human ZAQL-2), which is a mammalian peptide of frog Bv8 having a similar structure to that of human ZAQ ligand-1, activates MAP kinase and PI-3 kinase to exert a neuroprotective action (Eur. J. Neuroscience, 13, 1694, 2001). Subsequently, these peptides were reported also as novel peptides, prokineticin-1 (PK-1) and prokineticin-1 (PK-2) found through the DNA database (Mol. Pharmacol., 59, 692, 2001).
It is reported that human ZAQL-1 as an endocrine tissue-specific endocrine gland-derived vascular endothelial growth factor (EG-VEGF) takes part in the formation of highly permeable endothelial structure having a high permeability (fenestration), which characterizes the endocrine tissue, and that the recognition site of hypoxia-inducible factor-1 (HIF-1) involved in expression induction under hypoxic conditions is present in the transcription regulatory region of human ZAQL-1 gene to induce gene expression under hypoxic conditions (Nature, 412, 877, 2001).
To further clarify the physiological function of human ZAQL-1, a simple and high-sensitivity assay system for detecting/quantifying human ZAQL-1 has been earnestly desired.